How Do Fuses and Fuse Boxes Work? (2024)

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Bob Formisano

Bob Formisano

Bob Formisano is a licensed architect and builder with nearly 40 years of experience building new homes and restoring older homes. One of his specialties is repairing old systems dating back to the 1920s, including galvanized water pipes, knob-and-tube wiring, and more. His home repair articles for The Spruce span more than 10 years.

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Updated on 03/16/23

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Larry Campbell

Reviewed byLarry Campbell

Larry Campbell is an electrical contractor with 36 years of experience in residential and light commercial electrical wiring. He worked as an electronic technician and later as an engineer for the IBM Corp. He is also a member of The Spruce Home Improvement Review Board.

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Jillian Dara

How Do Fuses and Fuse Boxes Work? (2)

Fact checked byJillian Dara

Jillian is a freelance journalist with 10 years of editorial experience in the lifestyle genre. She is a writer and fact checker for TripSavvy, as well as a fact-checker for The Spruce.

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How Do Fuses and Fuse Boxes Work? (3)

Afuse box is a type ofelectrical service panel, which is a sort of control boardfor the entire electrical system of a house. While any home built around 1960 or later has a service panel full of circuit breakers, panels in older houses used fuses to provideover-current protectionfor the householdcircuits.

A fuse box has a seriesof threaded sockets into which the fuses are screwed in like light bulbs. Each circuit in the home is protected by a fuse, and each fuse must be the correct type and have an appropriate amperage rating for its circuit. Using the wrong type of fuse for a circuit can pose a serious fire hazard, so it's important to identify the correct fuse for each circuit.

  • 01 of 05

    Screw-in Fuse Bases

    Fuses for standard circuits (not high-voltage appliance circuits) are called plug fuses and have screw-in bases. There are two different types of bases and screw-in fuses: the Edison base (found on Type T fuses) and the rejection base (found on Type S fuses).

    The Edison base (Type T)looks like a light bulb base and fits the standard sockets found in old fuse boxes. Rejection base (Type S) fuses will work with Edison-type sockets only when combined with an adapter base that screws and locks into the Edison socket. The Type S fuse then screws into the adapter.

    Rejection bases are also known as "tamper-proof," and they were developed to prevent homeowners from using the wrong type of fuse for a circuit.Each Type S fuse of a specific amperage rating has a matching base adapter with a specific size of thread that prevents mismatching the fuses. For example, it stops a person from putting a 20-amp fuse in a 15-amp circuit, a potentially serious mistake. This condition is called over-fusing and can result in the fuse failing to blow before the circuit wiring overheats and potentially catchesfire.

    A 15-amp Type S fits only a 15-amp base adapter. By contrast, a Type T fuse can fit into any Edison socket, regardless of the circuit's amperage. If you have an old fuse box with Edison sockets, switching tosocket adapters and Type S fuses makes the panel much safer.

  • 02 of 05

    Type-W Fuses

    Type-Wfuses arean older style of fuse utilizing an Edison base and are all but obsolete today. They are general-purpose plug fuses and are fast-acting—that is, they have no time-delay fuse element and quickly interrupt the circuit once the fuse's rated amperage is exceeded.

    These fuses are designed for use in general lighting and power circuits that do not contain electric motors. Electric motors draw additional current at startup and will blow a Type W fuse if the motor is of any significant size. Because of this, time-delay fuses are used much more commonly than type-W fuses.

    Type-W fuse rating: 120 volts; up to 30 amps

  • 03 of 05

    Type-SL and Type-TL Fuses

    SL and TL fuses are medium-duty time-delay fuses and are now the most commonly used plug fuses found in home electrical systems. The only difference between SL and TL fuses is the type of base: the SL fuse has a rejection base, and the TL fuse has an Edison base.

    SL and TL fuses containa plug of heat-absorbing solder that's attached to the center of the fuse element (the part that burns out, or blows, during a circuit overload). This allows the fuse to absorb a temporary circuit overload, such as that caused by a brief surge in power demandwhen a motor starts up. Without a time-delay feature, simply starting your garbage disposer or refrigerator would cause a fuse to blow.

    Type SL and TL fuse rating: 120 volts; up to 30 amps

  • 04 of 05

    Type-S and Type-T Heavy-Duty Time-Delay Fuses

    Heavy-duty time-delay fuses are used for circuits withcritical or high motor loads or circuits serving motors that frequently cycle on and off (such as a sump pump motor). These fuses have a longer time-delay feature than the SL or TL fuses. However, just like the SL and TL fuses, the only difference between the S and the T heavy-duty fuses are the bases: type-S has a rejection base; type-T has an Edison base.

    Heavy-duty time-delay fuses contain a spring-loaded metal fuse link attached to a solder plug. If the overloaded circuit condition continues for too long, the solder plug melts and the spring pulls the fuse link free, cutting power to the circuit. This allows the fuse to absorb a longer temporary circuit overload than with other time-delay fuses.

    Type S and T heavy-duty fuse rating: 120 volts; up to 30 amps

    Continue to 5 of 5 below

  • 05 of 05

    Mini-Breaker Fuse

    Mini-breakers fuses are retrofit circuit breaker fuses that screw into Edison-base fuse sockets. They essentially replace a fuse with a push-button circuit breaker. Mini breakers have a little button that pops out when the circuit is overloaded. All you need to do is push the button back in to reset the breaker. Mini-breakers are also designed for time delay, so they do not trip unnecessarily when motors or appliances start up.

    Mini-breaker fuse rating: 120 volts; up to 20 amps

The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

  1. Cordero, Ismael. Checking and replacing fuses.Community eye healthvol. 25,78 (2012): 37. PMID:23139454

How Do Fuses and Fuse Boxes Work? (2024)


How Do Fuses and Fuse Boxes Work? ›

The conductor inside the fuse is made of a metal similar to solder. It has a lower melting point than the wire itself. The size of the conductor is calibrated very carefully so that when the rated current is reached, enough heat is generated to melt the conductor and so break the circuit.

How do fuse boxes work? ›

Fuse boxes are metal boxes that store fuses, which is an electrical safety measure that will shut-off power when the power is beyond that fuses ability to contain the power. Fuse boxes are very simple, yet intelligently designs devices that work by powering the electrical current through a piece of metal strip.

How does a fuse work in short answer? ›

A fuse consists of a piece of wire made of a metal or an alloy of an appropriate melting point, for example, aluminum, copper, iron, lead etc. If a current larger than the specified value flows through the circuit, the temperature of the fuse wire increases. This melts the fuse wire and breaks the circuit.

What is fuse answers? ›

A fuse is a service that delivers usable electrical circuit over-current safety. Some of the main fuse functions are as follows: Acts as a barrier between both the human body and the electric circuit. Inhibits system failure due to defective operation of circuits. Fuse avoids short-circuits.

What are fuses and circuit breakers and how do they work? ›

Fuses and circuit breakers are both designed to interrupt the flow of electricity. But they operate through different mechanisms. The fuse works as a piece of metal that melts down when overheated. While a circuit breaker works by operating a switching mechanism when an overflow of electricity is detected.

Are fuse boxes illegal? ›

Is it illegal to have an old fuse box? No, you're not going to be hauled off to jail if your home has an old fuse box. It's definitely legal to have one; however, your home may not be up to code. The National Fire Protection Association publishes the National Electrical Code, which has been adopted by all 50 states.

How does an electrical box work? ›

Power lines feed electricity to your breaker box via underground lines or power poles through a service drop. Once the electricity reaches your box, it is sent through branch circuits to power everything in your home. The main function of your breaker box is to protect your home from electrical overloads.

What is electric fuse short answers? ›

An electrical fuse is a safety device that operates to provide protection against the overflow of current in an electrical circuit. An important component of an electrical fuse is a metal wire or strip that melts when excess current flows through it.

What is a fuse in simple words? ›

A fuse is a safety device in an electric plug or circuit. It contains a piece of wire which melts when there is a fault so that the flow of electricity stops. The fuse blew as he pressed the button to start the motor.

Is fuse a switch? ›

Switch: Allows manual control to isolate a circuit for maintenance or in case of a fault. Fuse: A sacrificial device that melts or breaks when the current exceeds a predetermined limit, cutting off the power supply.

What does this fuse do? ›

If more electricity flows through a fuse than it was designed for, the fuse heats up so much that it melts. This opens a gap in the circuit, which stops the flow of electricity and protects the more expensive components from damage.

How does the fuse work? ›

In electronics and electrical engineering, a fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby stopping or interrupting the current.

How does a circuit breaker work simple terms? ›

The circuit breaker is an electromechanical device whose function is to protect electrical installations, being used in electrical distribution boards. It works by interrupting the electric current when it exceeds its design limitations, therefore preventing the supply of energy to the loads, and damage to the circuit.

What is the difference between a fuse box and a breaker box? ›

Both panels protect from overheating and fires. Circuit breakers protect against short circuits, but fuse boxes do not. Fuse boxes offer fast protection from malfunction and overcurrent. Circuit breakers perform best in modern homes requiring increased amperage.

Where does the fuse box get its power from? ›

The fuse box is powered directly by the battery.

What happens when a fuse blows in a fuse box? ›

The thin metal strip breaks apart when the fuse blows, terminating the electrical connection. Sometimes, the metal strip will also melt and discolor the glass window. If that happens, you might notice a burnt smell when you approach the fuse box, also known as an electrical panel.


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